When creating a drop shadow for objects, what is the traditional / classic "rule" for the offset direction? I see so many that are down and right, which results in the light source coming from the top left.

However, I read somewhere (I don't remember where, maybe an old David Siegel post from years ago) that when adding drop shadows to text, the shadow offset should be down and left, which provides a slight dark edge to the leading edge of the text, enhancing readability.

Or is the rule just "be consistent"?

Just to be clear, I'm asking if there's a design "principle" that has been established through tradition or precedence.

4 Answers
4

In your designs of your own devising, be consistent. If one object has a top-left light source, then all of your objects should have a top left light source.

In designs of others's devising, you need to be consistent with their format. In the case of Apple's Mac OS X (per the HIG), drop shadows come from the top-center, and actually a bit on the front around a 45˚ angle, and not from the side.

As for the direction, that is largely dependent on the effect you are trying to achieve, but as an informal rule based on not-so-casual observation, the light-source is typically on the top-left casting a bottom-right shadow. David Siegel may technically be correct, but it is almost entirely subjective in the end.

From kindergarten to elementary school the majority of my (and other kids') drawings had a thing called "corner Sun" which was a quarter of the Sun filling the upper-left corner. At first this analogue might sound far–fetched or tongue–in–cheek, but I think the concept of corner Sun points to the source where the idea of ideal drop shadow comes from.

Now, I was about to post a Google image search on the topic to enhance my point, but noticed a notable amount of drawings having the corner Sun in the upper-right corner! So generalizing directions ought to be taken with a pinch of salt* . I have a hunch that there might be either cultural or even left/right handedness connections—but I have no studies concerning the origins of the corner Sun. Definitely would read those.

What is the most common, however, is that the light is coming from upper left.

I think the main reason for this isn't derived from the concept of an ideal drop shadow, but from the cold fact that Photoshop has the default drop shadow direction set to 120° by default or to be more specific: the global light value is set to 120°. This looks adequate for most cases so why bother questioning the value every time. Adobe's reasoning for this very angle value would be interesting to hear.

I would just add as a side note, the difference between top lighting, i.e., the Sun, which has a friendly, pleasant connotation in our minds, and bottom lighting, i.e., fire or flashlights, which have a sinister, or evil correlation.
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SamApr 2 '11 at 12:42

@Sam Good point. It's like the halo of the saints vs. the usage of flashlight when telling horror stories at the campfire.
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koiyuApr 2 '11 at 13:26

That's really interesting Sam, and a good metaphor to remember. @koiyu, yeah, I wonder how this manifests in other cultures.
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ybakosApr 2 '11 at 13:54

Although I checked Philips answer for succinctness, I really like koiyu's answer too.
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ybakosApr 2 '11 at 13:58

I come more from the 3D CG realm, and when doing 3D work, you are attempting to create something that very much resembles reality. Rarely do shadows consistently fall in the same direction, not only because the shape of the objects can alter shadows a bit, but also because no light source behaves that way since their light is emitted radially from a single point in space -- or at least, in the case of a light bulb, a relatively small object, and the objects in view represent a significant variance in their respective direction from the light source. The only reason you don't see this type of behavior from the Sun is that the angular difference between two objects that are 10-ft apart here on Earth represent an indistinguishable difference in their direction from the Sun. The Sun and the Moon are the only common examples of this phenomenon in which the angle of the incoming light vector is consistent across our field of view, but light is so diffuse and scarce at night that you might as well not even talk about "consistent shadow angles" at night. Every day, we are presented with hundreds, if not thousands, of examples where shadows are not consistently cast in the same direction due to significant variance in incident light vectors or otherwise irregular lighting. (Examples: http://hebbylaya.deviantart.com/art/Stone-3d-Design-102879441 [d shadow off to left, n shadow off to right], http://fc00.deviantart.net/fs47/i/2009/216/4/b/3D_by_luiggi26.jpg (L shadow off to left, k, TM, and 3D shadow off to the right)

In Photoshop, however, reality is irrelevant. Name one thing in Photoshop that's realistic, and I bet I can find something wrong with the algorithm. In fact, the major reason to use the "consistent" shadows is precisely because Photoshop is nearly incapable of doing anything else. And don't get me wrong, I don't think the shadows that you see on the web look terrible, but they're not realistic either.

Even if Photoshop really could make realistic shadows, starting tomorrow, I don't think most people would use them because they wouldn't "look right" since we have already had certain ideas about what "looks right" for a web site or a logo engrained within us.

The bottom line is do what "looks right" because pretty much none of this stuff is realistic.

BTW, here's an example of a web site that is neither consistent nor realistic, but it still looks perfectly fine: http://us.battle.net/wow/en/services/ The "HOME/GAME/COMMUNITY/MEDIA/..." menu has a long highlight going across the top, but the entire centered div has a thin diffuse shadow around it, in all directions (if the light source were truly at the top, there should be no shadow at the top.) Furthermore, there's a little sub division under the menu that you might call the "content div". It's embossed a little, but instead of a long highlight at the top, it has two highlights at either corner, and then magically disappears after about 30 pixels. Completely inconsistent upon close inspection, but it looked perfectly fine at first glance!